One of the many steps involved in the fabrication of integrated circuit packages is wire bonding of the semiconductor die contact pads to the lead frame or chip carrier package. Particularly, once a semiconductor die has been fully fabricated and diced from the wafer, it is mounted onto a lead frame or chip carrier package. At this point, the contact pads on the surface of the die must be electrically coupled to the pins of the lead frame so that signals may pass to and from the die to external circuitry through the pins on the lead frame. The pins will protrude from the final molded integrated circuit package within which the die will ultimately be encased during fabrication. Generally, each contact pad is electrically coupled to one pin on the lead frame by means of wire bonding.
Wire bonding machines electrically connect the contact pads on the die to the electrical paths (i.e., pins) on the lead frame by placing a jumper wire which runs from the contact pad on the die surface to the conductive path on the lead frame. One widely used method of wiring bonding utilizes gold wire for the jumper wires.
In such wire bonding machines, a wire delivery system is provided. In one particular design, several hundred feet of continuous, wound gold wire is provided on a spool which is mounted on a spool holder on the wire bonding machine. The wire is run from the spool over a tensioner, through a wire clamp and into a capillary head. The clamp is in a fixed relation to the capillary head such that, when the clamp is closed (i.e., clamped), the wire is in fixed relation to the capillary head and, when the clamp is open, the wire may move through the capillary head.
In a modern, fully automated, wire bonding machine, lead frames with dies mounted thereon are (hereinafter lead frame/die units) run through wire bonding machines at a rate typically in the range of 1-20 lead frame/die units per minute depending on the number of bonds per unit. When a lead frame/die unit moves into position for wire bonding, a pattern recognition system locates the exact position of the die and lead frame. The capillary head is then positioned over the first contact pad of the die with the end of the gold wire at the tip of the capillary head. An electric flame off probe is positioned near the end of the wire protruding from the tip of the capillary head and applies an electric pulse or arc, termed an electric flame-off pulse, to the wire causing the end of the wire to melt into a ball. This electrical pulse typically may be on the order of approximately 5000 volts at approximately 10 milli-amps. As will be explained in greater detail below, the electric flame off pulse is grounded through the wire. Surface tension effects cause the melted wire to form into a ball. The ball is allowed to cool for a moment and solidify. The bond head then moves into position over the next pad to be bonded. With the wire clamp open, the capillary is then accelerated downward. The clamp closes en route to hold the gold ball protruding from the capillary head in place at the recess in the capillary tip. Finally, the capillary comes into contact with a contact pad on the die, at which time the clamp opens. The machine then bonds the ball to the contact pad by thermo-sonic bonding, i.e., applying ultrasonic energy at a temperature above ambient, for example 200.degree. Celsius.
The wire clamp, is open during the welding operation and remains open while the capillary head moves up trailing the wire out of its end. The head is moved upwards a distance sufficient to cause an amount of wire to trail out of the end of the capillary that can reach from the contact pad to the appropriate point on the lead frame to which that contact pad is to be electrically coupled via the gold wire. The clamp then closes and the capillary head moves to the point over the lead frame where the other end of the jumper wire is to be bonded. The capillary head is accelerated downward to cause the gold wire to contact the appropriate point on the lead frame. The wire is then thermo-sonically welded to the lead frame in the same manner as described above with respect to bonding to the die contact pad.
The clamp opens and the capillary head moves upwards trailing more wire. The clamp then closes and the head is moved up further to snap the wire at the second bond fracture point leaving a "tail" of wire protruding from the capillary. The tail wire will be formed into the next ball when the next electric flame off pulse or arc is applied. The capillary head then moves to a position above the next contact pad on the die and the above described process is repeated for each contact pad that is to be wire bonded to a pin on the lead frame. A modern wire bonding machine typically may be capable of making about 4-20 wire bonds per second on a die/lead frame unit.
In a fully automated fabrication facility, thousands of dies can be run in a single day comprising tens of thousands of wire bonds. It can be seen that wire bonding machines are an integral part of a semiconductor fabrication line and that failure of such equipment could potentially hold up an entire fabrication line. Accordingly, it is desirable to minimize breakdown and wear of wire bonding machines.
As mentioned above, the high voltage electric flame off pulse which melts the gold wire into a ball is grounded through the gold wire itself in the wire bonding machine. However, it is undesirable to allow the current pulses to run all the way through the wire into the spool because the current pulses running through the large number of turns in the wire wrapped around the spool would create a large, pulsing magnetic field due to induction effects. Such a magnetic field could have significant adverse effect on other electrical components in the area, including the electrical components of the wire bonding machine itself and the integrated circuits that are being fabricated. Accordingly, in many wire bonding machines, the current pulse is grounded from the wire through the wire clamp to ground so that the current does not run through the wire that is still on the spool.
Many wire bonding machines also have monitoring systems used to constantly determine whether the wire has broken at an unexpected time or in an unexpected place. Such systems commonly run a very low current from the tip of the capillary through the wire and the clamp to ground. A current sensor monitors the current between the clamp and ground. When the wire is not broken between the clamp and the capillary tip, the detection system will detect current. However, if the wire breaks between the capillary tip and the clamp, the current will stop flowing, thus tripping a warning system which will stop the machine.
Thus, two of the primary functions of the clamp are 1) to clamp the wire at programmed times, and 2) to provide a current path to ground through the wire. Consequently, two of the requirements for the material at the face of the clamp that contacts the wire are that 1) it should be hard and smooth so as to clamp and hold the wire securely without damaging it, and 2) it should be highly conductive.
One typical type of wire clamp comprises two clamp halves which close and open to clamp and unclamp the wire. One of the clamp halves is comprised of a hard and smooth, but not conductive, material such as sapphire or aluminum oxide (AlO.sub.2) The other clamp half commonly may be comprised of a cemented carbide material, such as tungsten carbide cemented with cobalt (WC/Co), titanium carbide cemented with nickel (TiC/Ni) or titanium carbide cemented with cobalt (TiC/Co). In particular, taking WC/Co as an example, the clamp half is comprised of tungsten carbide grains cemented together with a transition metal such as cobalt. The cementing component might also comprise nickel, either alone or in combination with cobalt. The overall metallurgic alloy typically might be approximately 93% WC and 7% Co. The metallurgical composition also may have some other small trace compounds such as titanium.
When the electric flame-off (EFO) pulse occurs and the current runs through the wire and clamp to ground, micro sparking occurs at the wire/clamp interface. Particularly, at the microscopic level, the surface of the WC/Co clamp is rough because it is comprised of WC granules bonded together by Co. Accordingly, at the microscopic level, sparks jump through the air from the wire to the WC/Co. Because WC has a resistivity substantially higher than that of Co, most of the current travels through the Co binder material in the clamp half. The micro sparking causes cobalt oxides to form on the surface of the clamp half. Cobalt oxides are not particularly conductive. The formation of the cobalt oxides on the surface causes current to be concentrated elsewhere where less cobalt oxide has formed. This causes increased micro sparking in those areas, thus even further increasing the rate of production of cobalt oxide. Accordingly, as cobalt oxide builds up, the problem compounds itself.
This corrosion of the cobalt further leads to failure of the WC/Co material because cobalt oxide is chalky, hydroscopic and wears off quickly, causing the WC granules to fall off. Accordingly, the micro sparking and related problems of cobalt oxide formation and material failure leads to two significant problems. First, the conductivity of the clamp decreases, leading to decreased efficiency in dumping the current from the EFO. It also leads to increased occurrence of false detections of wire breaks because of substantially diminished current flow due to the high resistivity of the clamp. Secondly, the problem of the WC grains falling off causes the clamp surface to become rough and to nick the wire, thus leading to wire breakage.
Even further, cobalt can alloy with gold to form a brittle, hard, material with low adhesion and with a coefficient of expansion that is large compared to cobalt. Accordingly, formation of a cobalt/gold alloy on the surface of the clamp is possible and the different coefficients of expansion could cause the clamp surface to crack.
Due to these various corrosion problems, conventional clamps require frequent cleaning, typically are one of the highest wear components of a wire bonding machine, and require frequent replacement.
Accordingly, it is an object of the present invention to provide an improved wire bonding machine.
It is another object of the present invention to provide an improved clamp for a wire bonding machine.
It is a further object of the present invention to provide a clamp comprised of a highly conductive, hard, smooth material that will minimize corrosion due to the wire bonding operation.
It is yet a further object of the present invention to provide a clamp for a wire bonding machine comprised of a material which overcomes the aforementioned problems of the prior art.